1. Field of the Invention
The present invention relates to a superconducting solenoid and a method of making the same, and more particularly, to a winding arrangement for such a superconducting solenoid which is capable of improving superconductivity stability, which is of construction sturdy enough to effectively prevent degradation of superconductivity, and in which satisfactory conduits for a cooling medium are ensured.
2. Description of the Prior Art
FIG. 4 is a cross section showing a superconducting solenoid made in accordance with a conventional "wind and react" procedure, which is described in literature such as, for example, in a paper entitled "High-Field Magnet formed of New Nb.sub.3 Sn Wires", by Koizumi et al, issued in May 1978 in preparation for the Twentieth Low-Temperature Engineering Conference. In this Figure, a superconducting coil 102 is wound around a coil-winding frame or core 101 in the form of a cylinder. As clearly illustrated in FIGS. 5A and 5B, the superconducting coil 102 is made by winding around the frame 101 wires 103 of filamentary conductors each covered with an electrical insulator 104 formed of a heat-resisting material such as glass fibers, heat treating the wires 103 thus wound around the frame 101 to produce superconductors, and impregnating a resinous material 105 into spaces formed between the windings so as to obtain a sturdy winding construction.
Now, description will be made of the coil-making procedure and the effects resulting therefrom.
In general, superconductors including compounds such as Nb.sub.3, V.sub.3 Ga or the like are extremely brittle and hence it is difficult to effect winding of wires of such superconductors after they are heat treated to form electric wires. This is because the allowable strain of these superconductors is less than 1 percent, as described in a publication entitled "Proceedings of a NATO Advanced Study Institute on the Science and Technology of Superconducting Materials (1980), page 474. Accordingly, a typical conventional process generally employed is that, as shown in FIG. 5A or 5B, wires 103 formed of metal composites, which do not cause compound-forming reactions, are each enclosed by an insulator 104 of a heat-resisting material and are wound around the coil-winding frame 101. As the insulator 104, it is usual to employ fibers of heat-resisting glass of high purity (so-called S glass) so as to give it heat resistivity enough to withstand the heat treatments as described later in detail.
After being wound in the above manner, the wires 103 are placed in a furnace and burned there at about 800.degree. C. so as to form an intermetallic compound, and thus superconductors are obtained which can be put into practical use. The superconductors formed of Nb.sub.3 Sn or V.sub.3 Ga have a so-called transition temperature of 18.degree. K. or therearound, at which superconductivity is lost, the transition temperature being higher than those of other kinds of superconductors. Therefore, it is generally cosidered that stability in superconductivity of the Nb.sub.3 Sn or V.sub.3 Ga superconductors is extremely high. However, the coil formed of the superconductors thus obtained has a loose structure so that it can not be put into practical use. The reason for this is that with the loose construction of the coil, the windings of wires 103 are permitted to move relative to each other, thereby readily destructing or quenching the superconductivity of the coil particularly under conditions where the stability in superconductivity of the coil is relatively limited. To improve this situation, the coil is carefully treated such that a resinous material is impregnated under vacuum into spaces formed between the windings so as to completely fill the voids around the windings. If this treatment is effected completely, it is possible to increase coil current to the critical level inherent to the superconductors.
With the conventional superconducting solenoid of the so-called "wind and react" type as produced in the above-described manner, cooling of the entire solenoid by means of a cooling medium is insufficient and thus the critical superconducting current of the solenoid, even if equivalent to that obtained with bare wires, is very unstable. In order to improve stability in superconductivity, there is no choice but to either effect satisfactory cooling of the solenoid or to reduce current densities. In this case, however, the requirements of the former choice can not be satisfied and therefore the latter choice has to be made with the result that the size and hence the production cost of the solenoid are considerably increased.